An internal combustion engine configured to gain thermal efficiency by means of providing a expansion stroke longer than the compression stroke was invented by James Atkinson in 1882, and is known as the Atkinson cycle. In 2002, Toyota Motors employed the Atkinson cycle on their “Prius” gasoline-electric hybrid automobile by configuring the intake valve timing for late valve closure during the compression stroke. A disadvantage of the Atkinson cycle approach to enhance expansion ratio is that engine volumetric efficiency is reduced by the reduction in combustion chamber charge volume, which increases the weight per unit horsepower of the engine. For a vehicle application, the resultant increased engine weight leads to an increase in overall vehicle weight, which detracts from the goal of reducing overall vehicle fuel consumption. The present invention avoids this weight disadvantage by providing a means to selectively engage compound mode operation to increase expansion ratio, without affecting maximum engine power capacity when compound operating mode is de-selected.
The Curtiss Wright R-3350 turbo-compound radial airplane engine of the 1950's recovers exhaust gas energy, which would otherwise be wasted, by means of multiple power recovery turbines coupled through gearboxes to the engine output shaft. This solution minimizes the weight penalty of adding exhaust gas expansion cylinders, however the high cost of the power recovery turbines and their associated gearboxes has since precluded application of the power recovery turbine method for non-aviation use. The present invention avoids this turbine and gearbox cost penalty by configuring the engine's own cylinders to act as selective power recovery expansion cylinders.
The method of improving the efficiency of a piston engine through compounding by utilizing a second cylinder to further expand working gas exhausted from a first cylinder has been widely applied to piston steam engines since the early nineteenth century. This same well known multiple cylinder compounding principle is applicable to internal combustion engines.
U.S. Pat. Nos. 6,202,416 Gray, 5,199,262 Bell, 4,917,054 Schmitz, 4,250,850 Ruyer, 4,237,832 Hartig and 4,159,700 McCrum, describe multiple cylinder compounding applied to an internal combustion piston engine, similar to the principle traditionally employed for compounded steam engines, by dedicating some of the engine's cylinders as exhaust gas secondary expansion cylinders, and describe valve timing and cylinder motion timing methods to effect the transfer of exhaust gas from fuel burning fired cylinders to secondary expansion cylinders. The Bell patent also describes a separate crankshaft for the expansion cylinders, driven at twice crankshaft speed, for the purpose of reducing the required size of the expansion cylinders, reducing the weight penalty of the expansion cylinders as compared to the McCrum, Schmitz and Gray patents. However, even with the reduced expansion cylinder size, the addition of dedicated expansion cylinders according to these prior patents adds significant weight and bulk to the engine, which is counterproductive to the goal of reducing vehicle fuel consumption. The present invention differs from the Gray, Bell, Schmitz and McCrum patents in that, according to the present invention, the expansion cylinder can selectively change back and forth, while the engine is running, from functioning as an expansion cylinder to functioning as a conventional fired cylinder, whereas, according to the Gray, Bell, Schmitz and McCrum patents, the function of the expansion cylinders is fixed, such that they are unable to function as conventional fired cylinders.
In addition, the present invention provides a means to store a compressed charge of exhaust gas in an exhaust gas expansion chamber and an exhaust manifold until any such time as the expansion cylinder is ready to accept it, whereas, according to the Gray, Bell, Schmitz, Ruyer and McCrum patents, the stroke timing of the power and expansion cylinder pistons must be constrained to a specific relative crankshaft clocking angle in order to facilitate the transfer of the exhaust gas charge from the fired cylinder to the expansion cylinder.
The present invention requires at least two conventional poppet type exhaust valves in each expansion cylinder head, similar to the Hartig patent and similar to one variant of the Ruyer patent. However, in the case of these Hartig and Ruyer patents, one of the exhaust valves in each cylinder only functions when compound mode operation is selected and remains closed when all cylinders are firing. With an inactive valve occupying part of the cylinder head, there is less port area available for the functioning valves, which constrains port size, thereby detracting from volumetric efficiency and increasing engine specific weight. The present invention retains use of all the exhaust valves when all the cylinders are firing, thereby imposing no penalty on maximum power capacity.
U.S. Pat. Nos. 7,121,236 Scuderi and 6,789,514 Suh describe a split cycle engine configuration in which intake and compression takes place in a dedicated cylinder, then the compressed gas charge is transferred to a second fired cylinder in which the charge is burned, expanded and exhausted. Such a split cycle engine may be configured with a charge cylinder having smaller volumetric displacement than the combustion cylinder, thereby increasing expansion ratio and improving thermal efficiency. However, such a split cycle cylinder configuration incurs the same overall engine weight penalty as does the Atkinson cycle configuration because of the consequent reduction of total engine volumetric efficiency. The present invention differs from the Scuderi and Suh patents in that, according to the present invention, the expansion cylinder is not used for combustion while compound operating mode is selected, rather it is used for secondary expansion of completely burned combustion products gas provided by a separate fuel burning fired cylinder.
Cylinder deactivation is a known method of improving the efficiency of a spark ignition engine operating at moderate power output, as further described by U.S. Pat. No. 7,260,467, Megli, and SAE Technical Paper Jan. 26, 2003. General motors applied cylinder deactivation to production Cadillac car engines in 1981. With this known method, dis-engageable couplings of conventional design are provided as part of the valve train for some of the cylinders, which when selected, de-couple the affected valves from their respective valve drive cams, causing the affected valves to remain closed, thus preventing fresh charge air from entering or leaving the deactivated cylinders. Fuel to the deactivated cylinders is shut off by an automatic controller. In the traditional method, the deactivated cylinders repeatedly compress and expand a trapped air charge within the cylinder. The remaining engine cylinders function normally as fired cylinders. A consequent reduction in total air flow to the engine allows the intake throttle valve to be opened wider to maintain the same moderate amount of power output. The resulting reduction in charge air pressure drop across the throttle valve eliminates some of the charge air throttling losses, resulting in an estimated five to ten percent increase in part-power engine efficiency for this cylinder deactivation method, with no adverse affect on the engine's maximum power rating. Similar dis-engageable valve drive cam couplings comprise components of the present invention, and the present invention also gains efficiency benefits from reduced throttling losses, however, the present invention differs from the cylinder deactivation method in that the affected cylinder or cylinders do not function as deactivated cylinders, instead these cylinders actively expand combusted gas discharged from one or more fired cylinders.
U.S. Pat. No. 4,401,069, Foley, describes an improvement on the cylinder deactivation principle in which an axially moving camshaft can selectively shift between two cam profiles for each valve, without stopping the engine. Similar cam profile selectivity comprises a part of the present invention, however, like the Megli patent, the Foley patent facilitates only cylinder deactivation, whereas the present invention utilizes selective cam profile changing in order to facilitate the active expansion of combusted gas discharged from one or more power cylinders.
Individual working elements comprising the present invention may appear conventional, however, in the present invention these working elements combine according to a new operating principle which has not been contemplated in the prior art.
Notwithstanding the numerous prior systems contemplated for addressing efficiency losses associated with the conventional four stroke cycle engine, and in light of the increasing cost and scarcity of petroleum based motor fuel, there remains a need for a simple, low cost, and light weight method for recovering otherwise wasted exhaust gas energy during moderate engine power operation, without adversely affecting the engine's maximum power rating.